The ultimate goal of this proposal is to evaluate the changes induced in the tumor microenvironment upon treatment with oncolytic viruses (OV) and assess how these changes impact OV therapy. These results will lead to a better understanding of OV therapy induced changes in tumor biology. OV treatment of tumors relies on cancer-specific replication of the virus leading to tumor destruction with minimal toxicity to adjacent no- neoplastic tissue. Results from clinical trials using replication competent OVs to treat patients with malignant gliomas have shown the new modality to be relatively safe, but high expectations of efficacy remain unmet (1, 2). The tumor's microenvironment is increasingly recognized as an important determinant for its progression and its response to therapeutics. We are investigating changes in the tumor microenvironment following oncolytic viral (OV) therapy, with the ultimate goal to devise better treatment strategies to combat brain tumors. In our preliminary studies, we have uncovered a significant increase in the release of HMGB1. HMGB1 is normally a cellular protein which is released into the extracellular environment upon certain stimuli and can serve as a damage associated molecular pattern. In this study we will evaluate the impact of this extracellular HMGB1 on tumor microenvironment, its impact on endothelial cells (Aim 1), macrophages (Aim 2) and antigen presentation to T cells (Aim 3).